The 40-year-old was having lunch with friends in her garden when she was hit by searing head pains and Paltrow thought she was having a stroke.
Doctors later found the actress had suffered a migraine and a panic attack, and she was subsequently told she was severely anaemic and vitamin D deficient, reported Sun Online.
The actress was ordered to change her diet and has written a new cook book, modelled by her new eating regime, called 'It's All Good: Delicious, Easy Recipes That Will Make You Look Good and Feel Great'.
"I got a searing pain in my head, I couldn't speak, and I felt as if I couldn't breathe. I thought I was having a stroke," she writes in the book.
Paltrow raises two kids with husband Chris Martin.